Generally, the manufacturing process for the automotive body includes stamping, welding, coating and assembling. In the stamping process which is the first step of the manufacturing process, drawing, trimming and flanging are carried out by passing through 3 to 4 stamping dies. The formation defects such as fractures, buckling and the like which occur during the drawing step give influence to the later processes. As a result, the quality of the final products is deteriorated, and the productivity is lowered, thereby leading to the increase of the manufacturing cost. The drawing step of the stamping process, which plays the critical role in forming the automotive body will be described referring to the attached drawings, for the case where a single acting press is used. As shown in FIG. 1, a steel sheet 5 to be subjected to a formation is inserted into between a lower die 3 and an upper die 1 on which a draw bead 4 is installed. Then the upper die 1 is lowered, so that it should give a proper force to the steel sheet by the help of the reaction force of a cushion 6 which supports the lower die 3. At the same time, as shown in FIG. 2, the lower die 3 is lowered down to the depth of the panel to be formed. As the lower die 3 is lowered, the steel sheet 5 which is positioned between the upper die 1 and the lower die 3 passes through the draw bead 4 to be put into the upper die 1, so that the steel sheet should be formed in accordance with the shape of a punch 2, thereby completing the stamping process. However, as shown by the portion A of FIG. 2, fractures can occur on the wall of the formed panel during the drawing process. The occurrence of such fractures is very sensitively affected by the mechanical properties of the steel sheet, the design of the dies, and other stamping conditions.
The deformation which occurs to the steel sheet during the stamping process includes stretching and drawing deformations, and, in the former, the material is not permitted to be mobilized at the flange portion by the lock bead, while, in the latter, the mobilization of the material is accompanied in the flange portion. Meanwhile, the deformation mode in which the reduction of the thickness of the steel sheet occurs in connection with the fractures during the stamping process includes a bi-axial tensile deformation mode, and another deformation mode in which the deformation in one-direction is inhibited, and the deformation in the perpendicular direction exists. About 75-90% of the fractures which occur during the stamping process belong to a plane strain mode in which the deformation in one direction is zero. Therefore, in order to prevent fractures during the stamping process and to forecast the stamping formability, it is desirable to evaluate the stamping formability, i.e., the formability limit of the steel sheet under the plane strain mode.
There is a conventional method for evaluating the formability under the plane strain mode without considering the frictions between the die and the steel sheet. According to this conventional method, the steel sheet is formed into a tensile test piece having a shape such that the test specimen has multi-stepped widths. Then a tensile test is carried out to realize a plane strain mode, and then, the formability under the plane strain state is evaluated based on the tensile properties such as the elongation to the fracture. This conventional method cannot take into account the frictions occurring between the die and the steel sheet due to the surface condition of the steel sheet. Further, the plane strain state occurs locally on the central portion of the tensile test specimen, and a considerable time and caution are required in preparing the test specimen.
Meanwhile, there is a conventional method for evaluating the stretch formability under the plane strain mode, with the contact between the die and the steel sheet being taken into account. That is, as shown in FIG. 3, a rectangular test specimen 7 which has a constant width and has a length longer than a lock bead 8 is inserted into between a lower die 10 and an upper die 9 on which the lock bead 8 is installed. Then the circumferential edge of the test specimen is strongly clamped by means of the lock bead 8, so that the material should not flow into the upper die 9. Then a dome shaped punch 11 having a diameter of 101.6 mm is elevated to apply the stretch formation force to the test specimen 7. Thus, the limit dome height LDH at the instant of the fracture of the test specimen 7 is recorded, thereby assessing the stretch formability under the plane strain mode.
In this method, there is realized a geometrical restriction in which the test specimen 7 having a diameter larger than that of the dome shaped punch 11 and smaller than that of the lock bead 8 surrounds the peak portion B of the dome shaped punch 11. Further, there is obtained a plane strain state in which the deformation in the direction of the width of the test specimen 7 and around the fracture area C of the test specimen 7 is zero as shown in FIG. 4. According to this method, the plane strain state is realized only around the fracture area C, but the peak portion B of the punch does not represent the stretch formability under the plane strain mode in the stern standard of the bi-axial tensile state.
Further, in the case where there are differences in the surface roughness and other surface characteristics such as surface treatment among the test specimens, the width of the test specimens does not give constant values, and therefore, deviations are severe in the repeated tests for the height of the punch until a breaking occurs after the variation of the width. Further, in order to decide the width of the test specimen representing the plane strain mode, many rounds of repeated tests have to be carried out, and therefore, much time is consumed.